1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus for forming a latent image on a photosensitive material by an optical image from an illuminated original document.
2. Description of the Prior Art
In a conventional copying machine, a photosensitive drum having a photosensitive material such as a photosensitive layer on a surface thereon on which a latent image is to be formed has an entire area of the surface uniformly charged prior to the illumination by an optical image for the latent image formation. However, since a width of the photosensitive drum is wider than a width of a copy paper of a maximum possible copy size, when a copy paper of other than the maximum size is set or even when the copy paper of the maximum size is set, an area other than that corresponding to the copy paper on the photosensitive drum is charged. Thus, in the past, such a charge is eliminated prior to the development of the latent image in accordance with the size of the copy paper to prevent developing agent from being deposited to such area so that waste of the developing agent and scatter of the developing agent in the apparatus are prevented. However, if an original document of a size A4 is to be copied in the copying machine in which the copy papers of a size A3 are set, the waste and the scatter of the developing agent are prevented by discharging the unnecessary area in accordance with the size of the copy paper, nevertheless, dirt on an original document cover for fixing the original document, and dirt on a belt for conveying the original document to an exposure station, are copied on that area of the size A3 copy paper which does not correspond to the size A4 original document, as a result, a copied image is dirty.